Tuning the Circumference of Six-Porphyrin Nanorings

Most
macrocycles are made from a simple repeat unit, resulting
in high symmetry. Breaking this symmetry allows fine-tuning of the
circumference, providing better control of the host–guest behavior
and electronic structure. Here, we present the template-directed synthesis
of two unsymmetrical cyclic porphyrin hexamers with both ethyne (C2)
and butadiyne (C4) links, and we compare these nanorings with the
symmetrical analogues with six ethyne or six butadiyne links. Inserting
two extra carbon atoms into the smaller nanoring causes a spectacular
change in binding behavior: the template affinity increases by a factor
of 3 × 109, to a value of ca. 1038 M–1, and the mean effective molarity is ca. 830 M. In
contrast, removing two carbon atoms from the largest nanoring results
in almost no change in its template-affinity. The strain in these
nanorings is 90–130 kJ mol–1, as estimated
both from DFT calculation of homodesmotic reactions and from comparing
template affinities of linear and cyclic oligomers. Breaking the symmetry
has little effect on the absorption and fluorescence behavior of the
nanorings: the low radiative rates that are characteristic of a circular
delocalized S1 excited state are preserved in the low-symmetry
macrocycles.